This invention relates to apparatus for transferring heat from a first environment to a second environment of lower temperature and is directed more particularly to devices known as heat exchangers.
Basically, a heat exchanger includes a wall of high heat conductivity metal separating two environments of different temperatures. If the wall is the shell of a device such as a rocket chamber, for example, which must be protected from destruction by the combustion inside the rocket chamber, a coolant medium or fluid may be directed or flowed over the outer surface of the wall to absorb the heat.
Other examples of heat exchangers include the automobile radiator which comprises a plurality of tubes through which there is directed the liquid coolant from an engine. The temperature of the coolant is reduced by air directed against the radiator tubes.
In the past, attempts to increase the heat transfer have included roughening the surface of the wall in contact with the flowing fluid, increasing the fluid velocity by decreasing the cross-sectional area of the coolant tubes or channels and/or increasing the pressure drop across each channel to increase fluid velocity. Additionally, the use of fins on the dividing wall or which extend radially inwardly or outwardly from the cooling tubes have been utilized.
Some of the problems which result from previous prior art attempts to increase the heat transfer include lack of control of surface roughness, low cycle thermal fatigue problems which result from joining different sections of material by welding or brazing or the like, the difficulty of machining suitable size channels in thin walls and confined spaces, and the difficulty of machining fins in small channels.